1. Field of the Invention
The present invention generally relates to a flame spray gun for feeding and melting a rod of heat fusible material and spraying molten droplets thereof onto a substrate to form a protective coating thereon and, more particularly, is concerned with a control apparatus coupled to the gun for controlling the rate of feeding of the rod of heat fusible material through the gun.
2. Description of the Prior Art
Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material, such as metal or ceramic, and propelling the softened material in particulate or droplet form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto to form a protective coating thereon. A thermal or flame spray gun is typically used for both heating and propelling the particles.
In the flame spray gun disclosed in U.S. Pat. No. 4,325,512 to Kenshol, the heat fusible material in rod form is fed into a combustion head by a pair of feed rolls driven by an air-driven turbine. The leading tip of the rod of heat fusible material is softened and melted by a flame in the combustion head and is atomized by an atomizing blast gas, such as compressed air, delivered to a spray head. The atomized material in finely divided particles or droplets is propelled from the spray head by the blast gas onto a surface to be coated. The spray head includes a spray nozzle and a gas cap for providing an annular flame around the axially fed rod of heat fusible material.
Several problems have been experienced with the flame spray gun of the above-cited Kenshol patent which appear to arise from the use of the air-driven turbine or motor and result in the production of a coating of poor quality. The first problem relates to the introduction of contaminants, such as dirt, oil and grease, into the gun with the air drawn in by the air-driven motor. Such introduction of contaminants adversely affects the speed and performance of the motor and produces flaws, such as pitting and unevenness, in the coating. The second problem relates to the variation in rate of feeding of the rod of heat fusible material produced by the inability to effectively control the volume and pressure of the air used to drive the motor. Such variation in feeding rate translates into variation in the rate of atomization of the rod which prevents the achievement of a consistent or uniform thickness in the coating produced on the surface being coated.
The thermal or flame spray gun disclosed in U.S. Pat. No. 5,275,336 to Stasi et al would appear to overcome the first problem described above relating to introduction of contaminants into the coating in that an electric motor is used instead of the air-driven turbine to drive the feed rolls to feed the rod of heat fusible material. The electric motor is not air-driven and so the introduction of contaminants by air is avoided.
However, the Stasi et al patent does not recognize the second problem and so does not disclose or suggest a solution to it. Consequently, a need still exists for improvement of the flame spray process as performed by use of a flame spray gun of the aforementioned type wherein the rate of feeding of the rod of heat fusible material through the gun is precisely controlled such that the result is the production of a precise high quality coating.